What is the purpose of the Central Polypurine tract (cPPT) on your viral vectors?

The Central Polypurine tract helps with transduction into non-dividing cells. Although no one really understands the mechanism by which the Central Polypurine tract helps translocation into the nucleus of nondividing cells at this time, it appears to allow the virus to penetrate the nuclear membrane.
Some links to literature:
http://www.bloodjournal.org/cgi/content/full/100/3/813
http://www.liebertonline.com/doi/abs/10.1089/104303401750214311?journalCode=hum